In the electrochemical production of chlorine and sodium hydroxide, brine containing sodium chloride and minor amounts of other salts, such as calcium sulfate, is fed to electrolytic cells. Since the calcium may interfere with cell operation, it is customary practice to precipitate the calcium with a carbonate, such as sodium carbonate. The calcium carbonate precipitate is removed from the brine before it is fed to the cell, but the sodium sulfate produced during such precipitation is carried into the cell along with the sodium chloride. Chlorine in gaseous form is taken off overhead from the cell and sodium hydroxide in the cell effluent is recovered in more concentrated form from evaporators. The cell effluent contains substantial amounts of sodium chloride which is customarily returned to the brine feed and recycled through the electrolytic cells.
The sodium sulfate has a decreased solubility in concentrated sodium hydroxide solutions, and precipitates out in the evaporators along with the sodium chloride. Sodium sulfate tends to build up on recycle through the evaporators and may cause decreased current efficiency in the electrolytic cell as well as high caustic losses during the separation of the caustic from the cell effluent. Consequently, it is desirable to remove as much of the sodium sulfate as possible from the sodium chloride before it is returned as brine feed to the electrolytic cells. It is also desirable that the sodium sulfate be removed in as concentrated form as possible in order to reduce capital expenditures necessary to further process the sulfate stream for discard or for production of a salable chemical. It is also desirable to remove all but a minor portion of the sodium hydroxide from the sodium chloride that is to be used as brine feed for the cells in order to maintain the proper pH level in the cells.
One way of approaching this overall problem, and a suggested process for solving it, is set forth in U.S. Pat. No. 4,087,253. This patent proposes using a caustic solution of less than 35% sodium hydroxide to effect separation of the sodium sulfate from the sodium chloride salt. While in theory this problem of separating sodium sulfate and sodium hydroxide from sodium chloride could be resolved by reference to equilibrium graphs or charts showing the solubility of these salts in aqueous solution, there are certain secondary phenomena, such as the formation of the triple salt of sodium hydroxide-sodium sulfate-sodium chloride, that prevent dependable prediction of optimum concentrations, temperatures and flow rates for the separation of these salts. Consequently, the solution to this problem of separating the sodium sulfate and sodium hydroxide from highly concentrated solutions or slurries of sodium chloride must depend upon empirical experimentation.